Fix a crash in the parser due to ParseStatementCore returning null (#19281)

Other call sites appear to have been adapted already.
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=266237

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -8222,7 +8222,7 @@ private LabeledStatementSyntax ParseLabeledStatement()
             var label = this.ParseIdentifierToken();
             var colon = this.EatToken(SyntaxKind.ColonToken);
             Debug.Assert(!colon.IsMissing);
-            var statement = this.ParseStatementCore();
+            var statement = this.ParseStatementCore() ?? SyntaxFactory.EmptyStatement(EatToken(SyntaxKind.SemicolonToken));
             return _syntaxFactory.LabeledStatement(label, colon, statement);
         }
 

src/Compilers/CSharp/Test/Syntax/Parsing/StatementParsingTests.cs

@@ -2705,6 +2705,23 @@ public void TestRunEmbeddedStatementNotFollowedBySemicolon()
             Assert.Equal((int)ErrorCode.ERR_SemicolonExpected, statement.Errors()[0].Code);
         }
 
+        [WorkItem(266237, "https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=266237")]
+        [Fact]
+        public void NullExceptionInLabeledStatement()
+        {
+            UsingStatement(@"{ label: public",
+                // (1,1): error CS1073: Unexpected token 'public'
+                // { label: public
+                Diagnostic(ErrorCode.ERR_UnexpectedToken, "{ label: ").WithArguments("public").WithLocation(1, 1),
+                // (1,10): error CS1002: ; expected
+                // { label: public
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "public").WithLocation(1, 10),
+                // (1,10): error CS1513: } expected
+                // { label: public
+                Diagnostic(ErrorCode.ERR_RbraceExpected, "public").WithLocation(1, 10)
+                );
+        }
+
         private sealed class TokenAndTriviaWalker : CSharpSyntaxWalker
         {
             public int Tokens;